1. Field of the Invention
The present invention relates to a subsea raw water injection facility for injecting sea water into an oil bearing formation.
There is a demand for new techniques to reduce the cost of exploration and production activity in off-shore oil fields such as the North Sea. As oil is discovered in ever deeper water, the costs of and risks associated with fixed platforms supporting production equipment increase. Thus anything which can be done to reduce the amount of equipment that has to be supported above the surface of the sea is useful.
2. Discussion of the Related Art
It is conventional practice to inject sea water into subsea oil bearing formations to assist with the process of sweeping oil from the formation and maintaining the pressure of the formation. Generally an oil production platform is provided with a processing plant to which sea water gathered from close to the sea surface is delivered. The processing plant typically comprises in series a coarse filter in the form of a screen, a fine filter, typically a body of sand, a chlorination unit, an oxygen scavenging unit and a de-oxygenation unit. The water is chlorinated to avoid biological activity and de-aerated to prevent rapid corrosion of pipes used to deliver processed water to an injection string leading to the subsea formation. The various sea water processing units are heavy and bulky and therefore supporting them above the surface of the sea is a significant problem, particularly in deep water.
International Patent Specification No. WO 94/29222 describes a sea water injection system in which the relevant processing units are mounted on the seabed. In the described system, a pump connected to an injection string is located within an enclosure the walls of which are porous. The pump is energized to draw sea water through the porous wall and appropriate arrangements are made to chlorinate the water as it passes through the wall. Particulates separated from the sea water passing through the wall will build up on the outside of that wall but it is believed that turbulence within the sea water will be sufficient to prevent the filter defined by the wall from being blinded.
The arrangement described in the published patent specification has its attractions in terms of simplicity but the concentration of particulates in sea water adjacent the seabed will be sufficiently high during periods of turbulence to make it difficult to predict the performance of the filter. Given the cost implications of an installed system failing there is a preference for using tried and test filtration systems which have a positive mechanism for discharging particulates separated from the pumped sea water.
A filter system is available which efficiently separates both mineral particulates such as sand and organic particulates which have neutral buoyancy. The known system incorporates an ejection mechanism which is periodically actuated so as to discharge separated particulates from the system. The use of such a system in the seabed environment is considered to be acceptable in terms of performance, but there is great concern about the long term viability of the particulate ejection system given that it is expected that large quantities of particulates will be separated and therefore the ejection system will have to be operated at regular intervals, for example many times each day.